Because of the snake's camouflage, I'll bet the bird didn't really realize its life was in danger even after the viper moved. If the bird was focused on the lure, it's very likely that it thought it was dodging some falling rocks or other inanimate object and returned to capture the prey when it felt safe. That's my best explanation because most birds will absolutely flip out as soon as they locate a snake and will warn other birds nearby, but this bird did not give any of the usual warning signals.